1. Field of the Invention
The present invention relates to a process for producing silicon semiconductor wafers which have a low density of as-grown defects.
2. The Prior Art
It is known that silicon wafers are cut from single crystals and are processed further to form the basic material for producing electronic components. The single crystals are usually produced using the Czochralski method (CZ method) or the float zone method (FZ method). In these methods, molten material, generally doped silicon, is solidified to form a single crystal that cools. In the CZ method, the single crystal is pulled from a melt that resides in a quartz glass crucible. This being the case, oxygen originating from the crucible material dissolves in the melt and is to some extent incorporated into the single crystal. The FZ method is a crucible-free pulling process. Thus, the oxygen concentration in float-zone single crystals (FZ crystals) is substantially lower than in single crystals pulled from crucibles (CZ crystals). There is, however, the possibility of doping FZ crystals with oxygen during their production. Thus, their oxygen concentration reaches values which are comparable with the oxygen doping in CZ crystals. A modified FZ method of this type is, for example, described in U.S. Pat. No. 5,089,082. The doping of FZ crystals with oxygen is, in particular, carried out in order to make the crystal lattice of the single crystal mechanically stronger. This is done also to use oxygen precipitates which, as so-called intrinsic getters, collect metallic impurities.
Neither CZ crystals nor FZ crystals have a perfect crystal lattice. The lattice contains ordering faults, which are referred to as as-grown defects. The term "defects" will be used hereafter exclusively to denote as-grown defects. For the production of electronic components, it is of prime importance for a semiconductor wafer to have the lowest possible defect density. This is particularly important in the region near the surface of the wafer. Any defect located in a region of a silicon wafer close to the surface can interfere with the functioning of an electronic component. This can even lead to failure of the component. The defect densities for FZ wafers are normally substantially lower than the defect densities which are found with CZ wafers. However, in the case of oxygen-doped FZ wafers having an oxygen doping concentration of at least 4*10.sup.17 /cm.sup.3, the defect densities reach values which are of the same order as the defect densities in CZ wafers. The doping of single crystals with oxygen, which is unavoidable in the case of CZ crystals, is often desired in the case of FZ crystals. This necessarily leads to high defect densities.
The connection between the defect density and the expected quality of the electronic components suggests the development of single crystals having low defect density. This is especially desirable since it is known that the defect density in a semiconductor wafer can be reduced by a heat-treatment referred to as annealing (M. Sano, M. Hourai, S. Sumita and T. Shigematsu, in Proc. Satellite Symp. to ESSDERC Grenoble/France, B. O. Kolbesen, Editor, p.3, The Electrochemical Society, Pennington, N.J. (1993)). Essential parameters during the annealing include the temperature, the annealing time, the atmosphere and the rates of change in temperature. The reduction in defect density is usually more pronounced for higher temperatures and longer annealing times. A disadvantage with this procedure is that long annealing times at high temperatures necessarily increases the production costs for the silicon wafers.
Research work has recently been published (D. Graf, U. Lambert, M. Brohl, A. Ehlert, R. Wahlich and P. Wagner, Materials Science and Engineering B36, 50 (1996)). This research discloses that the defect size plays a part when reducing defect densities by annealing. In addition, research has found that the rate at which a single crystal is cooled during its production affects the size distribution of the defects. However, this research work does not contain any indication of whether and how this discovery can advantageously be employed for the production of silicon wafers.